1. Field of the Invention
The present invention relates to a shift control system for a vehicle automatic transmission, which comprises engine load sensor means for producing a signal corresponding to the engine load; vehicle speed sensor means for producing a signal corresponding to an actual vehicle speed; and means responsive to said signals from both of said engine load and speed sensor means for selecting a reference shifting speed corresponding to an associated engine load from at least one predetermined relationship between the engine load and a plurality of reference shifting speeds, for comparing said selected reference shifting speed with the actual vehicle speed and for deciding whether shifting is needed or not.
The invention also relates to a shift control system for a vehicle automatic transmission, which comprises engine load sensor means for producing a signal corresponding to the engine load; vehicle speed sensor means for producing a signal corresponding to an actual vehicle speed; means responsive to the signals from said engine load sensor means and said vehicle speed sensor means and a signal indicative of an associated shift position of the automatic transmission for determining a programmed acceleration corresponding to the engine load, the actual vehicle speed and the associated shift position; means responsive to the signal from said engine load sensor means for producing an actual acceleration; and means responsive to said programmed acceleration and said actual acceleration for selecting a desired one of two or more kinds of predetermined relationships (shift patterns) between the engine load and a plurality of reference shifting speeds, a reference shifting speed corresponding to the relevant engine load given from the selected relationship between the engine load and the reference shifting speeds being derived and compared with the actual vehicle speed to decide whether shifting is needed or not.
2. Description of the Prior Art
In known shift control systems for automatic transmissions of this kind, particularly, in those shift control systems for the automatic transmission using a so-called torque converter, a problem is caused that a time delay is introduced, as shown in FIG. 1, in a relationship between depression of the accelerator pedal and the vehicle speed (i.e. the relationship between the torque and the vehicle speed) owing to slip in the torque converter and inertia of the vehicle, and thus, a reference shifting speed which is unduly low is selected, for example, the first speed is chosen when the second speed should be chosen.
Moreover, a shift control system has already been proposed, wherein the actual acceleration of the vehicle is picked-up from the output signal of the vehicle speed sensor, whereas, from a signal indicative of a shift position of the automatic transmission and the throttle opening and vehicle speed data from the engine load sensor and the speed sensor at that time, a programmed acceleration is determined which is to be attained with the associated shift position, throttle opening and vehicle speed under given running conditions, for example, a running condition on the road having a slope of 5% and that having a slope of 10%. Then, the programmed acceleration thus attained is compared with an actual acceleration detected by the speed signal from the vehicle speed sensor in order to determine continually or at a fixed time interval a desired shift pattern, for example, a running pattern for the flat road with its reference shifting speed being set to be low, a running pattern for the road of a small slope (of larger than or equal to 5%) with its reference shifting speed being set slightly high, or a running pattern for the road of a steep slope (of larger than or equal to 10%) with its reference shifting speed being set to be higher. In the known shift control systems of this type, however, a problem arises in that, when data from the throttle sensor and the speed sensor, in particular, are taken as instantaneous data, as shown in FIG. 1 by a pointer mark at each of the predetermined time intervals (t.sub.A) so that a shift pattern is determined at each predetermined time interval, a programmed acceleration which does not correspond to the true condition of running may be obtained due to a delay in the response of the vehicle speed relative to changes of the throttle opening. For example, in spite of a running condition on a flat road, a shift pattern for running on a slope is selected. Further, in this case, operations of the sensors in vehicles with much noise, vibration, and the like, become astable thereby, and upon reading data into memory means at the fixed time interval, incorrect input data may be read into the memory means. Particularly, when a throttle sensor of the potentiometer type is used, it may happen that its closed contacts are made accidentally open because of the vibration and even that no input data is read into the memory means. Consequently, there is a problem in that an incorrect shift pattern is selected based upon the erroneous input data as described above, whereby shifting which is not matched to the running condition of the vehicle is carried out. In particular, such a problem is encountered that when input data which is subject to fluctuations due to the vibration of the vehicle is read into the memory means at every fixed time interval in succession, a shift pattern to be selected is also caused to be varied at every fixed time interval in sequence, which may even lead to a sequence of sudden shifting, for example, sequential shifting of the second speed to the third speed and then to the second speed, so that the vehicle becomes very uncomfortable to ride in.
The principal object of the invention is to obviate, among the various disadvantages as described above, a particular one which is caused by the delay in the response of the vehicle speed relative to variations of the throttle opening. To this end, the shift control system according to the invention comprises means for extracting an item of speed data from the speed sensor, which appears at an instant delayed by a predetermined period of time from a corresponding item of throttle opening data and is used for a decision of necessity or non-necessity of shifting or for determination of a programmed acceleration instead of the speed data at the same instant as the corresponding throttle opening data.
Consequently, deviation of the speed data from the true running condition due to delay in response of the vehicle speed relative to depression of the accelerator pedal can be compensated for, and the disadvantages as mentioned above can be eliminated.
In such a shift control system wherein the shift pattern is determined by the programmed acceleration, the problems that any undesired shift pattern may be selected owing to variations of the throttle opening data and the speed data by vibration of the vehicle, etc. can be solved by averaging these data at every fixed time interval. In this case the problem which is caused by a delay of the response of the vehicle speed in relation to the depression of the accelerator pedal can be removed by delaying a time interval for detection of an averaged value of the speed from that for detection of an averaged value of the throttle opening by a predetermined amount of time.